The variable type does not align with the export type

Question

The variable type does not align with the export type

My TypeScript project includes a file that loads environment variables and exports them:

const.ts:

const {
  VARIABLE0, // type of VARIABLE0 is string | undefined
  VARIABLE1,
} = process.env;

if (!VARIABLE0 || !VARIABLE1) {
  throw new Error('Invalid env');
}

console.log(VARIABLE0); // type of VARIABLE0 is string
console.log(VARIABLE0.length);

export {
  VARIABLE0, // type VARIABLE0 is string | undefined
  VARIABLE1,
};

I am puzzled as to why the exported type is string | undefined even though I have checked for its existence. When I hover over VARIABLE0 after the check in VS Code, it shows as string, but during export, it is string | undefined. This inconsistency causes inconvenience in other parts of my code where I need to use these variables because I have to add ? and ! in numerous places (e.g.,

const length = VARIABLE0?.length!

instead of const length = VARIABLE0.length).

Is there a way to export it as string without the undefined part?

One approach I can think of is:

const {
  VARIABLE0: VARIABLE0_TEMP, VARIABLE1: VARIABLE1_TEMP,
} = process.env;

if (!VARIABLE0_TEMP || !VARIABLE1_TEMP) {
  throw new Error('Invalid env');
}

const VARIABLE0 = <string>VARIABLE0_TEMP;
const VARIABLE1 = <string>VARIABLE1_TEMP;

export {
  VARIABLE0, VARIABLE1,
};

However, this solution may not be optimal as it is quite verbose, especially since I have around 10 variables to export in a similar manner.

typescript

Answer 1

Answer №1

If you want to narrow down process.env, one approach is to utilize an assertion function. Here's an example:

function validateConfig<T extends string>(
    env: NodeJS.ProcessEnv,
    required: readonly T[],  // Using 'readonly' enables const assertion
): asserts env is NodeJS.ProcessEnv & Record<T, string> {
    const missing = required.filter((envVar) => !(envVar in env));
    if (missing.length > 0) {
        throw new Error(`missing required env vars: ${missing.join(", ")}`);
    }
}

Now you can apply this like so:

validateConfig(process.env, [
    "VARIABLE0",
    "VARIABLE1",
]);

Either an error will be thrown or both process.env.VARIABLE0 and process.env.VARIABLE1 will be of type string. Subsequently, you can do things like:

const { VARIABLE0, VARIABLE1 } = process.env;

export {
  VARIABLE0, // type VARIABLE0 is string
  VARIABLE1,
};

Check out this Playground

Note: When the required argument is typed as string[], the result will be

NodeJS.ProcessEnv & Record<string, string>

. This informs the compiler that all possible environment variables will exist - either explicitly list the required variables or use as const to obtain a more specific string union array/tuple type.

Answer 2

If you want to narrow down process.env, one approach is to utilize an assertion function. Here's an example:

function validateConfig<T extends string>(
    env: NodeJS.ProcessEnv,
    required: readonly T[],  // Using 'readonly' enables const assertion
): asserts env is NodeJS.ProcessEnv & Record<T, string> {
    const missing = required.filter((envVar) => !(envVar in env));
    if (missing.length > 0) {
        throw new Error(`missing required env vars: ${missing.join(", ")}`);
    }
}

Now you can apply this like so:

validateConfig(process.env, [
    "VARIABLE0",
    "VARIABLE1",
]);

Either an error will be thrown or both process.env.VARIABLE0 and process.env.VARIABLE1 will be of type string. Subsequently, you can do things like:

const { VARIABLE0, VARIABLE1 } = process.env;

export {
  VARIABLE0, // type VARIABLE0 is string
  VARIABLE1,
};

Check out this Playground

Note: When the required argument is typed as string[], the result will be

NodeJS.ProcessEnv & Record<string, string>

. This informs the compiler that all possible environment variables will exist - either explicitly list the required variables or use as const to obtain a more specific string union array/tuple type.

The variable type does not align with the export type

Answer №1

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